ci: detach HEAD before fetching origin refs

git refused 'fetch into branch ci/forge-mirror checked out at ...'
because the workflow runs on that branch and the same fetch refspec
overwrites its own ref. Detach HEAD first so no branch is current
and the fetch + force-update is allowed.

Author: rudyfraser

.github/workflows/tangle.yml

@@ -37,6 +37,10 @@ jobs:
           # the destination instead of as proper branches. Materialize them
           # under refs/heads/* first so the mirror push lands them as real
           # branches that forge can render.
+          #
+          # Detach HEAD before fetching so the fetch can overwrite
+          # refs/heads/<currently-checked-out-branch> without git refusing.
+          git -c advice.detachedHead=false checkout --detach
           git fetch origin '+refs/heads/*:refs/heads/*' '+refs/tags/*:refs/tags/*'
           git remote add tangled git@knot.blacksky.community:blackskyweb.xyz/rsky
           git push --mirror tangled